Semiconductor devices, such as thermistors, diodes and LEDs, require an air-tight encapsulation. In the past, a lead glass has been used for air-tightly encapsulating semiconductor devices, but recently, a lead-free glass has also been known, which is introduced in Patent Document 1 or 2. For a glass used for a semiconductor encapsulation, a glass raw material is melt in a melting furnace to form the molten glass in a tube shape, and then, the obtained glass tube is cut to a length of about 2 mm, and washed to produce a short glass tube which is referred to as a bead. Subsequently, by an inspection, the glass tube with defects or cracks are removed, and then shipped. Further, in an assembly of a diode, there is a case where the glass is exposed to an acidic plating solution or a flux in order for terminal treatment.
For the glass for semiconductor encapsulation, the following characteristics are required: (1) to be able to encapsulate semiconductor devices at a low temperature which does not deteriorate them, (2) to have a thermal expansion coefficient corresponding to the thermal expansion coefficients of metallic wires which input and output signals to and from semiconductor devices as well as the semiconductor devices, in order to ensure a reliable adhesion, (3) to have a sufficiently high adhesion between the glass and metal wires, (4) to have a high volume resistivity, (5) to have a sufficiently high chemical resistance, and the like.